Olefins, particularly ethylene and propylene, are desirable as a feed source for making derivative products such as oligomers, e.g., higher olefins, and polymers such as polyethylene and polypropylene. Olefin feed sources have traditionally been produced by cracking petroleum feedstocks.
U.S. Pat. No. 5,090,977 discloses a method of making olefins by steam cracking. The method includes separating the olefin product into methane, hydrogen, ethane, ethylene, propylene and C5+ streams. The disclosed separation preferentially produces propylene, and no propane, butane, butene, or butadiene streams are produced.
Oxygenate feed stocks, however, are becoming an alternative to petroleum feed stocks for making olefins, particularly large quantities of ethylene and propylene for the production of higher olefins and plastic materials. In general, the olefins are formed by contacting the oxygenate components with a molecular sieve catalyst to catalytically convert the oxygenates to olefins.
For example, U.S. Pat. No. 4,499,327, discloses a process for making olefins from methanol using any of a variety of silicoaluminophosphate (SAPO) molecular sieve catalysts. The process is carried out at a temperature between 300° C. and 500° C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hr−1. The process is highly selective for making ethylene and propylene.
U.S. Pat. No. 6,121,504 also discloses a method of making olefin product from oxygenate feed using molecular sieve catalysts. Water and other unwanted by-products are removed from the olefin product by contacting with a quench medium. After contacting with the quench medium, a light product fraction is obtained which comprises the desired olefins, but also includes dimethyl ether, methane, CO, CO2, ethane, propane, and other minor components such as water and unreacted oxygenate feedstock.
In order to further process olefins, it is often necessary to reduce or remove the amount of undesirable hydrocarbon by-products that are present in the olefin composition. This is because derivative manufacturing processes may use catalysts that are quite sensitive to the presence of certain hydrocarbons. For example, dimethyl ether has been found to act as a poison to certain catalysts.
U.S. Pat. No. 4,474,647 discloses that dimethyl ether can adversely impact the oligomerization of certain olefins. The patent describes a process for removing dimethyl ether from a C4 and/or C5 olefin stream using distillation. The stream is distilled and separated into an overhead and a bottoms stream. The overhead stream contains dimethyl ether, water, and various hydrocarbons, and the bottoms stream contains purified olefins.
U.S. Pat. No. 5,609,734 discloses a method of removing methanol and dimethyl ether from a mixed hydrocarbon stream. The hydrocarbon stream containing the methanol and dimethyl ether is distilled such that the dimethyl ether and methanol are removed in an overhead stream. Additional methanol is recovered in a side stream, in which a methanol permeable membrane is used to obtain the additional separation. Purified hydrocarbon is removed from an bottoms stream.
U.S. Pat. No. 4,387,263 discloses a method of making olefins from methanol using a zeolite catalyst. The methanol is contacted with the catalyst in a reactor to form an olefin product containing C2 to C4 olefins, water, unreacted methanol, and dimethyl ether, which is considered an intermediate product. The C2 to C4 olefins are separated from the water, unreacted methanol and dimethyl ether. The unreacted methanol and the dimethyl ether are then stripped from the water, and recycled to the reactor.
U.S. Pat. No. 4,603,225 discloses a method of making methyl tertiary butyl ether (MTBE) by reacting isobutylene with methanol in the presence of a catalyst. The MTBE is recovered by distillation, which also produces a raffinate stream. The raffinate stream contains unreacted olefins and methanol, water, and dimethyl ether. The dimethyl ether is washed from the raffinate stream using a methanol and water wash.
Removal of dimethyl ether from olefin streams is particularly difficult, since very low levels of dimethyl ether can act as catalyst poisons. This means olefin streams such as ethylene and propylene should contain little if any dimethyl ether, if further catalytic processing of the olefin stream is desired. Therefore, it is highly desirable to find additional methods of removing dimethyl ether from olefin streams.